The "Downtown Connector" (I-75 and I-85) in Atlanta, Georgia, United States of America, during rush hourAtlantacitizen/GFDL

Could the United States reduce greenhouse gas (GHG) emissions 80% from 2005 levels by 2050? A new report released this week says yes by assessing the potential for reducing petroleum consumption. The National Research Council report, “Transitions to Alternative Vehicles and Fuels” found that by the year 2050, the U.S. may be able to reduce petroleum consumption and greenhouse gas emissions by 80% for light-duty vehicles (cars and small trucks) through a combination of more efficient vehicles, the use of alternative fuels such as biofuels, electricity and hydrogen and strong government policies.

The most logical starting point, offering an economical and easy-to-implement approach, is improving the efficiency of conventional vehicles. However, improved efficiency alone will not meet the 2050 goals because the average fuel economy of vehicles on the road would have to exceed 180 mpg; a scenario the report says is extremely unlikely given current technologies. This is not to say that improved efficiency doesn’t play a role. “To reach the 2050 goals for reducing petroleum use and greenhouse gases, vehicles must become dramatically more efficient, regardless of how they are powered," said Douglas M. Chapin, principal of MPR Associates, and chair of the committee that wrote the report. Fuel efficiency measures center around decreasing the work the engine must perform, including: reducing vehicle weight, aerodynamic resistance, rolling resistance, and accessories as well as improving the efficiency of the internal combustion engine powertrain.

There are pros and cons to each of the scenarios that combine various alternative fuels and vehicles. For example, the study committee analyzed corn-grain ethanol and biodiesel biofuels, but found much greater potential in lignocellulosic biomass, which includes crop residues like wheat straw, switchgrass, whole trees, and wood waste. The beauty of this alternative fuel is that it can be used without major changes in fuel delivery infrastructure or vehicles.

Electric powered vehicles do not emit greenhouse gases, but the process of generating electricity often does so the report stresses the importance of successful carbon capture and storage. The additional load on the electric power grid is also a factor that must be considered. Furthermore, the batteries essential to these vehicles may limit the use of all-electric vehicles to local driving because of their close range and long recharge times. Serious technical challenges await advanced battery technologies under development.

Next the report considered using hydrogen as a fuel cell in electric vehicles. The pro is that the only vehicle emission is water; the con is that greenhouse gases are emitted during hydrogen production. There are low-greenhouse gas methods of making hydrogen, but they are currently expensive and require further development to become competitive. Another pro is that fuel cell vehicles do not have the same limitations as battery vehicles, but the con is the cost and difficulty entailed in revamping the current fuel infrastructure to fuel cells.

"Alternative fuels to petroleum must be readily available, cost-effective and produced with low emissions of greenhouse gases. Such a transition will be costly and require several decades. The committee's model calculations, while exploratory and highly uncertain, indicate that the benefits of making the transition, i.e. energy cost savings, improved vehicle technologies, and reductions in petroleum use and greenhouse gas emissions, exceed the additional costs of the transition over and above what the market is willing to do voluntarily," said Chapin. So to address the barriers to implementation of these technologies, the report suggested adaptive policies such as investment in research and development (R&D), subsidies, energy taxes or regulations to achieve the desired reductions.

The report cannot tell the future, but the best approach is to promote a portfolio of vehicle and fuel R&D. Both industry and government must support efforts to solve critical challenges. Meanwhile, evaluation should be ongoing to see which technologies emerge as the most promising and cost-effective.